470 Sir David Brewster on the Colours of Natural Bodies. 
dual, and those colours, though usually not very full, yet are 
often too full and lively to be of the fourth order.” 
Having thus determined that the green colour of vegetables 
must, according to this theory, be a green of the third order, 
we must inquire into its composition. Sir Isaac has himself 
stated, that the green of the third order “ is principally con- 
stituted of original green, but not without a mixture of some 
blue and yellow.” In point of fact, it consists of all the rays 
of the green space, with the least refrangible rays of the blue 
space, and the most refrangible rays of the yellow space, and 
it does not contain a single ray of zndigo or violet, nor a single 
ray of orange or red light. ‘This is its real composition, 
whether we deduce it from the theory of periodical colours, 
or obtain it by direct analysis with the prism. 
In order to discover the true composition of the green co- 
lour of plants, we may analyse the light which they reflect or 
transmit, but the best method is to extract the green colour- 
ing matter by means of alcohol, and to examine the action of 
the tingeing corpuscles when suspended in that fluid. For 
this purpose I have used the leaves of the common laurel, 
Prunus Lauro-cerasus, as a type of this class of colours. The 
leaves are torn into small shreds and put into absolute alcohol, 
and the fine green fluid which is thus obtained is either placed 
in a hollow prism with a large refracting angle, so as to ex- 
hibit its composition in its own spectrum, or the light trans- 
mitted through the fluid may be analysed by a fine prism, or 
the spectrum produced by such a prism may be viewed through 
a portion of the fluid bounded by parallel surfaces. By which- 
soever of these methods the experiment is made, we shall ob- 
serve a spectrum of the most beautiful kind. In place of see- 
ing the green space with a portion of dlwe on one side and 
yellow on the other, as the Newtonian theory would lead us 
to expect, we perceive a spectrum divided into several coloured 
bands of unequal breadths, and having their colours greatly 
changed by absorption. 
At a certain thickness of the green fluid there are three red 
bands. By increasing the thickness, the violet and blue spaces 
are absorbed, and the two inner red bands. An absorption 
then begins near the middle of the green space, and after de- 
stroying the more refrangible portion of that space, three 
bands are left; viz. one faint band of the extreme red, one 
band almost white, corresponding with the mest luminous 
spectrum, and one green band contiguous to the white one. 
In applying. this mode of examination to. the green co- 
lours of others plants, I have found them to have invariably 
the same composition. In the following list of plants of va- 
